1. Field of the Invention
The present invention relates to a high-frequency circuit, an optical module employing the same, and a method of matching the impedance.
2. Description of the Related Art
A conventional example of an optical module employing a high-frequency circuit is provided by an optical modulation device. The configuration of such a device is described in the literature ("Semiconductor Optical Switches/Modulators and Their Packaging" Kairo Jisso Gakkaishi [Journal of the Circuit Packaging Association] Vol.10, No.5, 1995, pp.306-309).
The conventional optical modulation device comprises a first lens (non-spherical lens), optical modulator (electroabsorption optical modulator) and a second lens (spherical lens) having mutually matching optical axes and arranged linearly on a carrier with a convex section.
Facing the electroabsorption optical modulator and intersecting its optical axis at right-angles are located on a high-frequency substrate a microstrip line and a terminating resistor. Connected to the microstrip line and fixed to the package is located a high-frequency connector.
The terminal of the optical modulator and the microstrip line are connected with the aid of bonding wire.
In the conventional optical modulation device, the high-frequency substrate which forms the microstrip line and the terminating resistor needs to be at least 1 mm wide, and it is impossible for the width of the carrier on which the optical modulator is mounted to be less than that of the highfrequency substrate. As a result, the conventional optical modulator is roughly the same width as the high-frequency substrate.
Moreover, the modulation zone of the electroabsorption optical modulator is restricted by the element capacity of this modulation device, and therefore element length is kept short (100 .mu.m or less) in order to ensure low capacity. Use of an optical modulation device of this type permits attainment of a modulation zone of around 40 GHz.
There are also reports of examples of the use of electroabsorption optical modulators with integrated waveguide where the modulator area is shortened and waveguide areas are provided on either side of it in order to prevent optical scattering of incident light.
However, these conventional optical modulation devices are fraught with problems as outlined below.
If the modulator elements are kept short in order to ensure low capacity, the abovementioned optical modulation devices which employ electroabsorption optical modulators or electroabsorption optical modulators with integrated waveguide show high frequency response characteristics of 40 GHz or above. This means that in any attempt to adopt them for use in frequency domains in excess of 40 GHz, it is extremely difficult to match the impedance between the high-frequency circuit containing the modulator on the one hand and other circuits, for instance, the high-frequency power supply circuit, driver circuit or amplifier circuit on the other hand.
It is an object of the present invention to provide a high-frequency circuit, optical module employing the same, and method of matching impedance which exhibit favourable characteristics even in frequency domains in excess of 40 GHz.